Now that you know how to navigate among channels and apply commands, permit me to suggest a few reasons for doing so. The most pragmatic applications for channel effects involve the restoration of bad color scans. If you use a color scanner, know someone who uses a color scanner, or just have a bunch of color scans lying around, you can be sure that some of them look like dog meat. (Nothing against dog meat, mind you. I'm sure that Purina has some lovely dog meat scans in their advertising archives.) With Photoshop's help, you can turn those scans into filet mignon — or, at the very least, into an acceptable Sunday roast.
The following are a few channel-editing techniques you can use to improve the appearance of poorly scanned full-color images. Keep in mind that these techniques don't work miracles, but they can retrieve an image from the brink of absolute ugliness into the realm of tolerability.
| Note |
Don't forget that you can choose Window Arrange New Window to maintain a constant composite view. Or you can click the eyeball icon in front of the composite view in the Channels palette to view the full-color image, even when editing a single channel. |
Aligning channels: Every so often, a scan may appear out of focus even after you use Photoshop's sharpening commands to try to correct the problem, as discussed in Chapter 10. If, on closer inspection, you can see slight shadows or halos around colored areas, one of the color channels probably is out of alignment. To remedy the problem, switch to the color channel that corresponds to the color of the halos. Then select the move tool (by pressing V) and use the arrow keys to nudge the contents of the channel into alignment. Use the separate composite view (created by choosing Window Arrange New Window) or click the eyeball in front of the composite channel to monitor your changes.
Channel focusing: If all channels seem to be in alignment (or, at least, as aligned as they're going to get), one of your channels may be poorly focused. Use the Ctrl-key (Win) or z -key (Mac) equivalents to search for the responsible channel. If you find it, use the Unsharp Mask filter to sharpen it as desired. You may also find it helpful to blur a channel, as when trying to eliminate moir patterns in a scanned halftone. (For a specific application of these techniques, see the "Cleaning up scanned halftones" section in Chapter 10.)
Bad channels: In your color channel tour, if you discover that a channel is not so much poorly focused as simply rotten to the core — complete with harsh transitions, jagged edges, and random brightness variations — you may be able to improve the appearance of the channel by mixing other channels with it.
Suppose that the blue channel is awful, but the red and green channels are in fairly decent shape. The Channel Mixer command lets you mix channels together, whether to repair a bad channel or achieve an interesting effect. Choose Image Adjustments Channel Mixer and press Ctrl+3 (z -3 on the Mac) to switch to the blue channel. Then raise the Red and Green values and lower the Blue value to mix the three channels together to create a better blue. To maintain consistent brightness levels, it's generally a good idea to use a combination of Red, Green, and Blue values that add up to 100 percent, as in Figure 4-24. If you can live with the inevitable color changes, the appearance of the image should improve dramatically.
Figure 4-24: Here I use the Channel Mixer command to repair the blue channel by mixing in 10 percent of the red channel and 30 percent of the green channel. The red and green channels remain unaffected.
Note that Channel Mixer is also a great command for creating custom grayscale images. Rather than choosing Image Mode Grayscale and taking what Photoshop gives you, you can choose the Channel Mixer command and select the Monochrome check box. Then adjust the Red, Green, and Blue values to mix your own grayscale variation.
Incidentally, the Constant slider simply brightens or darkens the image across the board. Usually, you'll want to leave it set to 0. But if you're having problems getting the color balance right, give it a tweak.
| Note |
Although the Channel Mixer didn't arrive until Photoshop 5, I created my own channel mixing filter quite a few years ago. Created in Photoshop's Filter Factory, this filter coincidentally went by the name . . . Channel Mixer! I submit Figure 4-25 as Exhibit A. "But Deke," you say, "Your filter doesn't look anything like Adobe's Channel Mixer, and your sliders don't make nearly as much sense." Yes, I imagine that's precisely what they want you to think. Perhaps now you're beginning to understand how diabolically crafty these Photoshop programmers can be. Figure 4-25: An early version of the Channel Mixer invented by yours truly. Has Adobe gone and swiped my visionary idea? You be the judge. |
The one channel function I've so far ignored is Image Mode Multichannel. When you choose this command, Photoshop changes your image so that channels no longer have a specific relationship to one another. They don't mix to create a full-color image; instead, they exist independently within the confines of a single image. The multichannel mode is generally an intermediary step for converting between different color modes without recalculating the contents of the channels.
For example, normally when you convert between the RGB and CMYK modes, Photoshop maps RGB colors to the CMYK color model, changing the contents of each channel as demonstrated back in Figures 4-17 and 4-18. But suppose, just as an experiment, that you want to bypass the color mapping and instead transfer the exact contents of the red channel to the cyan channel, the contents of the green channel to the magenta channel, and so on. You convert from RGB to the multichannel mode and then from multichannel to CMYK as described in the following steps.
Open an RGB image. If the image is already open, make sure that it is saved to disk.
Choose Image Mode Multichannel. This eliminates any relationship between the formerly red, green, and blue color channels.
Click the new channel icon at the bottom of the Channels palette. Or choose the New Channel command from the palette menu and press Enter or Return to accept the default settings. Either way, you add a mask channel to the image. This empty channel will serve as the black channel in the CMYK image. (Photoshop won't let you convert from the multichannel mode to CMYK with less than four channels.)
Press Ctrl+I (z -I on the Mac). Unfortunately, the new channel comes up black, which would make the entire image black. To change the channel to white, press Ctrl+I (z -I on the Mac) or choose Image Adjustments Invert.
Choose Image Mode CMYK Color. The image looks washed out and a tad dark compared to its original RGB counterpart, but the overall color scheme of the image remains more or less intact. This is because the red, green, and blue color channels each have a respective opposite in the cyan, magenta, and yellow channels.
Press Ctrl+Shift+L (z -Shift-L on the Mac). Or choose Image Adjustments Auto Levels. This punches up the color a bit by automatically correcting the brightness and contrast.
Convert the image to RGB and then back to CMYK. The problem with the image is that it lacks any information in the black channel. So although it may look okay on screen, it will lose much of its definition when printed. To fill in the black channel, choose Image Mode RGB Color, and then choose Image Mode Chapter 18).
Keep in mind that these steps are by no means a recommended procedure for converting an RGB image to a CMYK image. Rather, they merely suggest one way to experiment with channel conversions to create a halfway decent image. You can likewise experiment with converting between the Lab, multichannel, and RGB modes, or Lab, multichannel, and CMYK.
If you truly want to abuse the colors in an RGB or a CMYK image, there's nothing like replacing one color channel with another to produce spectacular effects. Color Plate 4-2 shows a few examples applied to an RGB image.
In the first example, I used the Channel Mixer to replace the red channel with the green. I did this by setting the Output Channel to Red, changing the Red value to 0 percent and the Green value to 100 percent. The result is a yellow whale in a violet-blue sea.
To achieve the next example, I again started from the original RGB image and used the Channel Mixer to replace the blue channel with the green. The result this time is a blue whale against a deep blue background.
You can create more interesting effects by using the Channel Mixer to swap the contents of color channels. For example, in the third example of Color Plate 4-2, I swapped the contents of the red and blue channels to create a green whale in a burnt-orange sea. To accomplish this, I set the Output Channel to Red, set the Red value to 0, and set the Blue to 100. Then I switched to the blue channel by pressing Ctrl+3 (z -3 on the Mac) and set the Red value to 100 and the Blue to 0.
As a final test, I did the same sort of swap to the original image as described in the last paragraph, but substituted the green channel for the blue. I then skipped over to the blue channel by pressing Ctrl+3 (z -3 on the Mac) and dropped the Blue value down to 50 percent, giving me the fuchsia and yellow fellow pictured in the bottom example of Color Plate 4-2.